Chief among the Maiar of Valinor whose names are remembered in the histories of the Elder Days are Ilmarë, the handmaid of Varda, and Eönwë, the banner-bearer and herald of Manwë, whose might in arms is surpassed by none in Arda.
Headcanon about the dynamic between Mairon and Langon I talked about with @morelen. It´s kind of long so you can read it below the cut.
I imagine that Langon has been a servant of Melkor for longer than Mairon. Because of that Langon was trusted to be his mentor when Mairon first arrived in Utumno. He was not very happy about his new task of teaching this newbie. He had better things to do and was pretty standoffish and mean towards him. Mairon on the other hand admired him and saw him as kind of an older brother.
As Mairon quickly rose through the ranks Langon changed his attitude towards him and wasn’t so annoyed anymore. At this point when they are roughly at the same power level they work very well together. Mairon sees Langon as his equal, the both of them being Melkor’s most trusted servants. Langon on the other hand appreciates Mairon for getting Melkor off his back.
When over time Melkor starts to favor Mairon over Langon their relationship starts to flip. As Langon desperately tries to stay relevant he is depending more and more on Mairon talking favorably about him with Melkor who in turn is starting to lose interest in him. Mairon eventually notices how Langon is losing his footing and remembers how Langon used to be so arrogant. He decided to not hold him over the water anymore, taking the position of Melkor´s right hand man for himself alone.
Mairon got to travel around with Melkor and oversee the construction of Angband while Langon was pushed to the side. Though the moment when he finally fell deep and hard only came when the Valar knocked on the doors of Utumno and he wasn’t able to stop them from entering (this element basically follows the plot from “the chaining of Melko”, the fourth chapter of the book of lost tales part 1). While Mairon was able to hide in Angband Melkor was chained and taken away.
After this event Mairon was basically on top of the food chain, his only possible contestor still being Langon. But Langon at this point was known for his weakened position and for being the one who opened the gates to the Valar (even tho it was under Mekors command) and practically kicking off this devastating battle. As a consequence Mairon chased him out and moved the remaining forces westwards to Angband while Langon fled eastwards.
